1. Field of the Invention
The present invention relates to a spindle motor, a method of manufacturing the spindle motor, and a magnetic disk drive having a spindle motor.
2. Description of the Related Art
In recent years, a so-called brushless DC servo motor is widely employed as a spindle motor used in a magnetic disk drive. In particular, in the magnetic disk drive, in order to decrease the size and increase the capacity of the magnetic disk drive, a so-called in-hub type spindle motor having a magnetic circuit portion in a hub that supports a magnetic disk is used to improve the space efficiency.
As such an in-hub type spindle motor, a motor having a double-sided support structure and a motor having a cantilevered support structure are provided. With the spindle motor having a double-sided support structure, a hub serving as a rotor is rotatably supported through a bearing by a stationary support shaft provided upright on the bottom wall of the magnetic disk drive, and the upper end of the support shaft is connected to the cover of the magnetic disk drive, thereby improving the rigidity of the stationary portions including the support shaft.
In the spindle motor having a cantilevered support structure, a bearing is provided in a cylindrical support projecting from the bottom wall of a magnetic disk drive, and a shaft integrally formed with the central portion of a hub that serves as a rotor is rotatably supported by the bearing.
Usually, the magnetic circuit portion of the spindle motor of these types is constituted by a back yoke, a magnet, a stator core, and coils, and is disposed between the inner circumferential surface of the hub and the bearing. The back yoke is integrally formed with the circumferential wall of the hub, and the magnet is formed annularly and is fixed to the inner circumferential surface of the hub.
Regarding the stator core, for example, a silicon steel plate having a thickness of 0.2 to 0.4 mm is press-cut to form a core member having a predetermined shape. A plurality of core members each obtained in this manner are stacked, thereby forming the stator core. The stator core has an inner cylindrical portion, a plurality of winding portions extending radially from the outer surface of the cylindrical portion, and wide end portions located at the extending ends of the respective winding portions. Slits are formed between adjacent winding portions. A coil is wound on the outer surface of each winding portion.
The upper and lower end portions of the coil wound on each winding portion project from the upper and lower surfaces, respectively, of the stator core. Hence, the effective thickness of the magnetic circuit portion becomes a sum of the projecting heights of the upper and lower end portions of the coil to the thickness of the stator core in the stacking direction. When designing a spindle motor, the stacking height of the stator core must be designed by considering the projecting heights of the end portions of the coils. In particular, when decreasing the size of the magnetic disk drive, the stacking thickness or height of the stator core is limited within a predetermined range as it largely influences the thickness of the entire spindle motor and the thickness of the magnetic disk drive.
To increase the capacity of the magnetic disk drive, the number of mounted magnetic disks must be increased. As the number of mounted magnetic disks is increased, the magnetic circuit efficiency, in particular the torque characteristics, of the spindle motor must be improved. Various methods are possible to improve the torque characteristics of the spindle motor, i.e., the torque characteristics of the magnetic circuit portion. Among these methods, one effective method is to increase the stacking thickness of the stator core, thereby increasing the motor constant.
More specifically, the motor constant is described by (torque constant)/(coil resistance).sup.1/2, and is proportional to the 1/2th or 3/4th power of the stacking thickness of the stator core depending on the width and stacking thickness of the stator core. Accordingly, to increase the stacking thickness of the stator core is effective in increasing the motor constant.
As described above, however, the space in the magnetic disk drive where the spindle motor can be set is limited, and the stacking thickness of the stator core is limited accordingly. Further, the heights of the end portions of the coil wound on each winding portion of the stator core are added to the effective stacking thickness of the stator core, making it further difficult to increase the stacking thickness of the stator core.
In particular, in a low-profile spindle motor, i.e., in a spindle motor having a stator core with a small stacking thickness, the projecting heights of the end portions of the coil occupy a large proportion in the height of the entire magnetic circuit portion. This serves as a major obstacle in increasing the stacking thickness of the stator core.
Due to the above reasons, the conventional spindle motor has varied problems when improving its torque characteristics, and it is difficult to improve the torque characteristics without increasing the entire thickness.